This disclosure relates to an aircraft jet engine mounted centrifugal fuel boost pump, for example, in particular to the impeller blades.
The boost pump is commonly packaged together with the main fuel pump, which is usually of a positive displacement gear pump type, both being driven by a common shaft. The fuel leaving the boost stage goes through a filter and a fuel oil heat exchanger before entering the main pump. Pressure losses are introduced by these components and the associated plumbing, while heat is also added to the fuel. The fuel feeding the boost pump comes from the main frame fuel tanks through the main frame plumbing. The tanks are usually vented to the ambient atmospheric pressure, or, in some cases, are pressurized a couple of psi above that. The tanks are provided with immersed pumping devices, which are in some cases axial flow pumps driven by electric motors or turbines, or in other cases ejector pumps, collectively referred to as main frame boost pumps.
During flight, the pressure in the tank decreases with altitude following the natural depression in the ambient atmospheric pressure. Under normal operating conditions, industry standards require the main frame boost pumps to provide uninterrupted flow to the engine mounted boost pumps at a minimum of 5 psi above the true vapor pressure of the fuel and with no V/L (vapor liquid ratio) or no vapor present as a secondary phase. Under abnormal operation, which amounts to inoperable main frame boost pumps, the pressure at the inlet of the boost stage pumps can be only 2, or 3 psi above the fuel true vapor pressure, while vapor can be present up to a V/L ratio of 0.45, or more. Definition of terms, recommended testing practices, and fuel physical characteristics are outlined in industry specifications and standards like Coordinating Research Council Report 635, AIR 1326, SAE ARP 492, SAE ARP 4024, ASTM D 2779, and ASTM D 3827, for example.
During normal or abnormal operation, the boost pump is required to maintain enough pressure at the main pump inlet under all the operating conditions encountered in a full flight mission such as the main pump can maintain the demanded output flow and pressure to the fuel control and metering unit for continuous and uninterrupted engine operation. There are also limitations in the maximum pressure rise the engine mounted boost pump is allowed to deliver such not to exceed the mechanical pressure rating of the fuel oil heat exchanger, or limitations pertaining to minimum impeller blade spacing such as a large contaminant like a bolt lost from maintenance interventions would pass through and be trapped safely in the downstream filter. All these requirements along with satisfying a full flow operating range from large flows during takeoff to a trickle of flow during flight idle descent, and fuel temperature swings from −40 F to 300 F, makes the aerodynamic design of the engine mounted fuel pumps a serious challenge.
In order to achieve the pressure rise demanded by the downstream main fuel pump and fuel metering system and to also be capable of operating with extreme low suction conditions encountered during the abnormal operation, the boost pump impellers are provided with a radial blade section and with an axial blade section upstream there from. The radial blade section is commonly referred to as the impeller blade section, while the axial blade section is referred to as the inducer blade section. The inducer's primary function is to sustain good pressure and flow conditions at the inlet of the impeller radial section even under the low suction conditions imposed by the abnormal operation, where the main frame boost pumps are inoperable.
The gap between the minimum required supply pressure for normal engine operation and the maximum allowed discharge pressure demanded by pressure rating limitations of the inter-stage fuel oil heat exchanger are often so narrow, that the final design is determined only after the first unit went through design and development testing. The impeller diameter, which primarily controls the pump pressure rise, is intentionally set to a slightly larger value in the initial design, the unit built and tested, and ultimately the impeller diameter is trimmed to its final value such to match all the constraints imposed by the requirements.